Synchronization in a beamforming system

ABSTRACT

A beamforming system synchronization architecture is proposed to allow a receiving device to synchronize to a transmitting device in time, frequency, and spatial domain in the most challenging situation with very high pathloss. A periodically configured time-frequency resource blocks in which the transmitting device uses the same beamforming weights for its control beam transmission to the receiving device. A pilot signal for each of the control beams is transmitted in each of the periodically configured time-frequency resource blocks. Pilot symbols are inserted into pilot structures and repeated for L times in each pilot structure. The L repetitions can be implemented by one or more Inverse Fast Fourier Transfers (IFFTs) with corresponding one or more cyclic prefix (CP) lengths.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119 from U.S.Provisional Application No. 62/054,485, entitled “Control Signaling in aBeamforming System,” filed on Sep. 24, 2014; U.S. ProvisionalApplication No. 62/054,488, entitled “Synchronization in a BeamformingSystem,” filed on Sep. 24, 2014, the subject matter of which isincorporated herein by reference.

TECHNICAL FIELD

The disclosed embodiments relate generally to wireless communication,and, more particularly, to control signaling and synchronization in aMillimeter Wave (mmW) beamforming system.

BACKGROUND

The bandwidth shortage increasingly experienced by mobile carriers hasmotivated the exploration of the underutilized Millimeter Wave (mmW)frequency spectrum between 3G and 300G Hz for the next generationbroadband cellular communication networks. The available spectrum of mmWband is two hundred times greater than the conventional cellular system.The mmW wireless network uses directional communications with narrowbeams and can support multi-gigabit data rate. The underutilizedbandwidth of the mmW spectrum has wavelengths ranging from 1 mm to 100mm. The very small wavelengths of the mmW spectrum enable large numberof miniaturized antennas to be placed in a small area. Such miniaturizedantenna system can produce high beamforming gains through electricallysteerable arrays generating directional transmissions.

With recent advances in mmW semiconductor circuitry, mmW wireless systemhas become a promising solution for real implementation. However, theheavy reliance on directional transmissions and the vulnerability of thepropagation environment present particular challenges for the mmWnetwork. In general, a cellular network system is designed to achievethe following goals: 1) Serve many users with widely dynamical operationconditions simultaneously; 2) Robust to the dynamics in channelvariation, traffic loading and different QoS requirement; and 3)Efficient utilization of resource such as bandwidth and power.Beamforming adds to the difficulty in achieving these goals. A robustcontrol-signaling scheme is thus required to facilitate the beamformingoperation in a challenging environment.

In cellular networks, pilot signals are needed for device identificationand time-frequency synchronization. Primary synchronization signal is aunique signal with smaller search space, which can be used for firststage synchronization to achieve coarse frame boundary and frequencysynchronization. Secondary synchronization signal is a unique signalwith larger search space, which can be used for second stagesynchronization to identify device and achieve fine (symbol level)timing and frequency synchronization. Reference signal is used forchannel estimation and demodulation of data symbols. The three types ofpilot signals for time-frequency synchronization and channel estimationintroduce too much overhead. Furthermore, spatial synchronization is notconsidered in existing solutions (e.g., LTE). Future systems operate inmuch higher carrier frequency band that requires beamforming with verynarrow beam width. As a result, synchronization signals need to alignwith TX and RX beams under spatial synchronization.

A beamforming system synchronization architecture is sought to allow thereceiving devices to synchronize to the transmitting devices in time,frequency, and spatial domains in the most challenging situation.

SUMMARY

A beamforming system synchronization architecture is proposed to allowthe receiving device to synchronize to the transmitting device in time,frequency, and spatial domain in the most challenging situation withvery high pathloss. A periodically configured time-frequency resourceblocks in which the transmitting device uses the same beamformingweights for its control beam transmission to the receiving device. Apilot signal for each of the control beams is transmitted in each of theperiodically configured time-frequency resource blocks. The samesynchronization signal can be used for all stages of synchronizationincluding initial coarse synchronization, device and beamidentification, and channel estimation for data demodulation. Pilotsymbols are inserted into pilot structures and repeated for L times ineach pilot structure. The L repetitions can be implemented by one ormore Inverse Fast Fourier Transfers (IFFTs) with corresponding one ormore cyclic prefix (CP) lengths. A detector at the receiving devicedetects the presence of the control beams, synchronizes to thetransmission and estimates the channel response by receiving the pilotsignals. The detector at the receiving device has low complexity whenexploiting the structure of the synchronization signals. It consists ofthree stages that break down the synchronization procedure into lesscomplicated steps. It accurately estimates the parameters required foridentifying the transmit device and performing subsequent datacommunication.

In one embodiment, a base station allocates a set of control resourceblocks in a beamforming OFDM network. The set of control resource blockscomprises periodically allocated time-frequency resource blocksassociated with a set of beamforming weights to form a control beam. Thebase station partitions each resource block into a pilot part and a datapart. Each pilot part is divided into M pilot structures and each pilotstructure comprises L OFDM symbols. Pilot symbols are inserted onceevery K subcarriers for R times in each of the L OFDM symbol to form thepilot part while data symbols are inserted in the remaining resourceelements to form the data part. The variables M, L, K, and R are allpositive integers. The base station then transmits the pilot symbols andthe data symbols via the control beam to a plurality of user equipments(UEs). The M pilot structures have a similar structure with a differentoffset v_(m) and a different sequence s_(m). Each control beam of a cellis identified by the pilot symbols having a hopping pattern based onv_(m) and a signature sequence s_(m). Specifically, for the j-th controlbeam of the i-th cell, there is a corresponding identifier pairv_(m)(i,j) and s_(m)(i,j)[n].

In another embodiment, a base station allocates time-frequency resourceblocks in a beamforming OFDM network for control beam (CB) transmission.The base station partitions each resource block into a pilot part and adata part. The pilot part comprises M pilot structures and each pilotstructure comprises a number of OFDM symbols in time domain and a numberof subcarriers in frequency domain. The base station then inserts pilotsymbols of a pilot signal in each OFDM symbol in the pilot part. Thepilot symbols are repeated for L times in each pilot structure, and eachpilot structure is applied by one or more Inverse Fast Fourier Transfers(IFFTs) with corresponding one or more variable cyclic prefix (CP)lengths for CB transmission. A user equipment (UE) receives control beamtransmission from the base station. The UE receives a time domain signalfrom pilot symbols that are transmitted over periodically allocatedtime-frequency resource blocks of a control beam. The UE processes thetime domain signal by removing one or more cyclic prefixes (CPs) with avariable CP length and performing corresponding one or morevariable-length Fast Fourier Transfers (FFTs) to reconstruct a pilotpart of a resource block, wherein the pilot part comprises M pilotstructures and each pilot structure comprises a number of OFDM symbolsin time domain and a number of subcarriers in frequency domain. The UEthen extracts the pilot symbols from each pilot structure. The pilotsymbols are repeated for L times in each pilot structure.

In yet another embodiment, a user equipment (UE) receives control beamtransmissions from a base station in a beamforming OFDM network. A pilotsignal is transmitted over periodically allocated time-frequencyresource blocks of a control beam in a cell. The UE processes pilotsymbols carried in a pilot part of a resource block, the pilot partcomprises M pilot structures and each pilot structure comprises L OFDMsymbols in time domain and R subcarriers in frequency domain. The pilotsymbols are inserted once every K subcarriers for R times in each OFDMsymbol, and M, L, R, and K are positive integers. The UE then detectsthe control beam and the pilot signal based on the control beamtransmission.

Other embodiments and advantages are described in the detaileddescription below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, where like numerals indicate like components,illustrate embodiments of the invention.

FIG. 1 illustrates control beams in a beamforming system in accordancewith one novel aspect.

FIG. 2 is a simplified block diagram of a base station and a userequipment that carry out certain embodiments of the present invention.

FIG. 3 illustrates beamforming weights applied to multiple antennaelements in a beamforming system.

FIG. 4 illustrates multiple sets of beamforming weights applied toantenna elements one beam at a time or two beams at a time.

FIG. 5 illustrates spatial reciprocity of DL and UL transmission in abeamforming system.

FIG. 6 illustrates control beams in a cell comprising DL controlresource blocks and UL control resource blocks.

FIG. 7 illustrates one embodiment of DL control resource blockassociated with a control beam.

FIG. 8 illustrates one embodiment of UL control resource blockassociated with a control beam.

FIG. 9 illustrates BS transmission and UE reception in DL controlresource blocks.

FIG. 10 illustrates UE transmission and BS reception in UL controlresource blocks.

FIG. 11 illustrates control beams in a cell comprising DL and UL controlresource blocks and their associated beamforming weights.

FIG. 12 illustrates control region, control region segment, and controlcycle of a control beam.

FIG. 13 illustrates control region segment and control resource blockconfiguration.

FIG. 14 illustrates a preferred embodiment of DL and UL control resourceblock configuration.

FIG. 15 illustrates an UL receiver having two RF chains for receivingtwo control beams simultaneously.

FIG. 16A illustrates embodiments with and without interleaved DL/ULcontrol resource configuration.

FIG. 16B illustrates one embodiment of control resource configurationwith different DL/UL duty cycles.

FIG. 17 illustrates embodiments of control cycles for different cells.

FIG. 18 illustrates embodiments of control cycles in TDD and FDDsystems.

FIG. 19 illustrates a control signaling procedure between a UE and a BSin a beamforming system in accordance with one novel aspect.

FIG. 20 is a flow chart of a method of control signaling from basestation perspective in a beamforming system in accordance with one novelaspect.

FIG. 21 is a flow chart of a method of control signaling from userequipment perspective in a beamforming system in accordance with onenovel aspect.

FIG. 22 illustrates one example of pilot signals in a control beam in abeamforming system in accordance with one novel aspect.

FIG. 23 illustrates a detailed example of pilot structures in controlbeams.

FIG. 24 illustrates control beam identification based on pilot signals.

FIG. 25 illustrates variations of pilot structures with additional OFDMsymbols.

FIG. 26 illustrates variations of pilot structures with guard time.

FIG. 27 is a flow chart of a method of allocating resources for pilotsymbol transmission in a beamforming system in accordance with one novelaspect.

FIG. 28 illustrates different embodiments of OFDM symbols withrepetitions in time domain.

FIG. 29 illustrates pilot structures with variable cyclic prefix lengthand variable length FFT.

FIG. 30 is a flow chart of supporting variable CP length for pilotsignal transmission in a beamforming network in accordance with onenovel aspect.

FIG. 31 is a flow chart of supporting variable CP length for pilotsignal reception in a beamforming network in accordance with one novelaspect.

FIG. 32 illustrates a three-stage pilot signal detection procedure inaccordance with one novel aspect.

FIG. 33 illustrates a stage-1 control beam detection and coarsetime-frequency estimation.

FIG. 34 illustrates a stage-2 control beam reference block boundarydetection.

FIG. 35 illustrates a stage-3 sequence correlation and beamidentification and fine time-frequency synchronization.

FIG. 36 is a flow chart of a method of pilot signal detection based oncontrol beam transmission in a beamforming network in accordance withone novel aspect.

DETAILED DESCRIPTION

Reference will now be made in detail to some embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 1 illustrates control beams in a beamforming Millimeter Wave(mmWave) cellular network 100 in accordance with one novel aspect.Beamforming network 100 comprises a base station BS 101 and a userequipment UE 102. The mmWave cellular network uses directionalcommunications with narrow beams and can support multi-gigabit datarate. Directional communications are achieved via digital and/or analogbeamforming, wherein multiple antenna elements are applied with multiplesets of beamforming weights to form multiple beams. For control purpose,a set of coarse TX/RX control beams are provisioned by the base stationin the cellular system. The set of control beams may be periodicallyconfigured or occur indefinitely and repeatedly in order known to theUEs. The set of control beams covers the entire cell coverage area withmoderate beamforming gain. Each control beam broadcasts a minimum amountof beam-specific information similar to Master Information Block orSystem Information Block (MIB or SIB) in LTE. Each beam may also carryUE-specific control or data traffic. Each beam transmits a set of knownsignals for the purpose of initial time-frequency synchronization,identification of the control beam that transmits the signals, andmeasurement of radio channel quality for the beam that transmits thesignals.

In the example of FIG. 1, BS 101 is directionally configured withmultiple cells, and each cell is covered by a set of coarse TX/RXcontrol beams. In one embodiment, cell 110 is covered by eight controlbeams CB0 to CB7. Each control beam comprises a set of downlink resourceblocks, a set of uplink resource blocks, and a set of associatedbeamforming weights with moderate beamforming gain. In the example ofFIG. 1, different control beams are time division multiplexed (TDM) intime domain. A downlink subframe 121 has eight DL control beamsoccupying a total of 0.38 msec. An uplink subframe 122 has eight ULcontrol beams occupying a total of 0.38 msec. The interval between theDL subframe and the UL subframe is 2.5 msec. The set of control beamsare lower-level control beams that provide low rate control signaling tofacilitate high rate data communication on higher-level data beams. Forexample, UE 102 performs synchronization with BS 101 via control beamCB4, and exchanges data traffic with BS 101 via dedicated data beam DB0.The control beam and data beam architecture provides a robustcontrol-signaling scheme to facilitate the beamforming operation inmmWave cellular network systems.

FIG. 2 is a simplified block diagram of a base station and a userequipment that carry out certain embodiments of the present invention.BS 201 has an antenna array 211 having multiple antenna elements thattransmits and receives radio signals, one or more RF transceiver modules212, coupled with the antenna array, receives RF signals from antenna211, converts them to baseband signal, and sends them to processor 213.RF transceiver 212 also converts received baseband signals fromprocessor 213, converts them to RF signals, and sends out to antenna211. Processor 213 processes the received baseband signals and invokesdifferent functional modules to perform features in BS 201. Memory 214stores program instructions and data 215 to control the operations of BS201. BS 201 also includes multiple function modules that carry outdifferent tasks in accordance with embodiments of the current invention.

Similarly, UE 202 has an antenna 231, which transmits and receives radiosignals. A RF transceiver module 232, coupled with the antenna, receivesRF signals from antenna 231, converts them to baseband signals and sendsthem to processor 233. RF transceiver 232 also converts receivedbaseband signals from processor 233, converts them to RF signals, andsends out to antenna 231. Processor 233 processes the received basebandsignals and invokes different functional modules to perform features inUE 202. Memory 234 stores program instructions and data 235 to controlthe operations of UE 202. UE 202 also includes multiple function modulesthat carry out different tasks in accordance with embodiments of thecurrent invention.

The functional modules can be implemented and configured by hardware,firmware, software, and any combination thereof. For example, from BSside, DL allocation module 221 and UL allocation module 222 allocatescontrol radio resource blocks for the control beams, and pilotallocation module 223 allocates radio resources for transmitting pilotsignals. Note that the term “allocate” can be an explicit actionperformed by the BS to configure and reserve certain resource blocks,but it can also be an implicit action of following a predefinedagreement based on a standard specification. From UE side, pilotdetection module 245 detects pilot signals, extract pilot symbols, andidentify control beams from received control beam transmission, beamselection module 244 selects a preferred control beam from receivedcontrol beam transmission, synchronization module 243 performs time andfrequency synchronization with the BS using the selected control beam,measurement module 242 measures radio signals for different controlbeams and cells, and random access module 241 performs channel accessfor establishing connection with the BS.

FIG. 3 illustrates beamforming weights applied to multiple antennaelements in a beamforming system. Through directional antennatechnology, complex beamforming weights are adjusted and then applied tothe signals transmitted or received by the multiple antenna elements tofocus the transmitting or receiving radiation power to the desiredirection. The beamforming weights W can be applied in analog domain inthe RF chain Nc (e.g., as illustrated in FIG. 3), or applied in digitaldomain at the baseband (not shown) depending on the transceiverarchitecture. Multiple sets of complex weights can be applied to themultiple antenna elements Nt, forming one beam at a time or multiplebeams simultaneously.

FIG. 4 illustrates multiple sets of beamforming weights applied toantenna elements to form one beam at a time or two beams at a time. Inthe top row of FIG. 4, the base station forms one beam at a time byapplying one set of weights. Beams 0, 1, 2, and 3 are sequentiallyformed one at a time. In the bottom row of FIG. 4, the base stationforms two beams at a time by applying two sets of weights. Beams 0/4,1/5, 2/6, and 3/7 are sequentially formed two at a time.

FIG. 5 illustrates spatial reciprocity of DL and UL transmission in abeamforming system. It is generally assumed that the downlink channeland the uplink channel is spatially reciprocal in the beamformingsystem. This is typically true for Time division duplex (TDD) systemsand for most Frequency division duplex (FDD) systems if the frequencyspacing is less than tenth of the total channel bandwidth. Underspatially reciprocal beamforming, the same beamformed antenna pattern isused for reception and transmission. As illustrated in FIG. 5, fordownlink transmission, the BS applies TX beamforming vector V_(BS,TX)and the UE applies RX beamforming vector V_(UE,RX). For uplinktransmission, the BS applies RX beamforming vector V_(BS,RX) and the UEapplies TX beamforming vector V_(UE,TX). Under spatially reciprocalbeamforming, the beamforming vectors for downlink and uplink are thesame, e.g., (V_(BS,TX), V_(UE,RX))=(V_(BS,RX), V_(UE,TX)).

FIG. 6 illustrates control beams in a cell comprises DL control resourceblocks and UL control resource blocks and associated beamformingvectors. As a general concept, a downlink control beam is defined as aset of time-frequency resource blocks in which the base station uses thesame beamforming weights set for its downlink transmission to thereceiving UEs. The said time-frequency resource blocks, referred to asdownlink (DL) control resource blocks, may be periodically configured oroccur indefinitely in order known to the UEs. The periodicallyconfigured downlink control resource blocks for downlink control beamCB0 is depicted in the top half diagram of FIG. 6, where V⁰ _(BS,TX)represents the beamforming vector for downlink CB0.

Similarly, an uplink control beam is defined as a set of time-frequencyresource blocks in which the base station preferably chooses the samebeamforming weights set as the one used by the corresponding DL controlresource blocks for its reception of the UEs' uplink transmission. Thesaid time-frequency resource blocks, referred to as uplink (UL) controlresource blocks, may be periodically configured or occur indefinitely inorder known to the UEs. The periodically configured uplink controlresource blocks for uplink control beam CB0 is depicted in the bottomhalf diagram of FIG. 6, where V⁰ _(BS,RX) represents the beamformingvector for uplink CB0. Because of spatial reciprocity, the beamformingvectors are the same (V⁰ _(BS,TX)=V⁰ _(BS,RX)). If the base stationchooses not to use the corresponding transmit beamforming weights set asits receive beamforming weights set in the UL control resource block,then it should use a beamforming weights set that achieves betterperformance than the beamforming weights set associated with the DLcontrol resource blocks.

FIG. 7 illustrates one embodiment of a DL control resource blockassociated with a control beam. Each DL control resource blockassociated with a control beam comprises at least a pilot part and adata part transmitted by the base station. For example, DL controlresource block 701 comprises pilot part 710 and data part 720. The pilotpart is used for identification of the cell and the control beam, andfor time, frequency, and spatial synchronization. The data part is usedfor cell-specific broadcast, beam-specific broadcast, UE-specificcontrol data, and UE-specific traffic data.

FIG. 8 illustrates one embodiment of an UL control resource blockassociated with a control beam. Each UL control resource block comprisesresources allocated to a certain UE. The transmission of a UE in the ULcontrol resource block comprises at least a pilot part and a data part.For example, UL control resource block 801 comprises resources 802 thatincludes pilot part 810 and data part 820. The pilot part is used foridentification of the UE, and for the base station to achieve time,frequency, and spatial synchronization to the UE's uplink transmission.The data part is used for UE-specific control data and UE-specifictraffic data.

The transmission of a UE in the UL control resource block may employtransmit beamforming when equipped with multiple antennas. The transmitbeamforming weights set used by the UE for the transmitting in the ULcontrol resource block should preferably be the same as the receivebeamforming weights set used by the UE for the reception in thepreceding DL control resource block with which the UL transmission isassociated.

FIG. 9 illustrates BS transmission and UE reception in DL controlresource blocks. FIG. 10 illustrates UE transmission and BS reception inUL control resource blocks. Under spatially reciprocal beamforming, thesame beamformed antenna pattern is used for reception and transmission.As illustrated in FIG. 9, for downlink transmission with control beamCB0, the BS applies TX beamforming vector V⁰ _(BS,TX) and the UE appliesRX beamforming vector V⁰ _(UE,RX) in DL control resource blocks. Asillustrated in FIG. 10, for uplink transmission with CB0, the BS appliesRX beamforming vector V⁰ _(BS,RX) and the UE applies TX beamformingvector V⁰ _(BS,TX) in UL control resource blocks. Under spatiallyreciprocal beamforming, the beamforming vectors of control beam CB0 fordownlink and uplink are the same, e.g., V⁰ _(BS,RX)=V⁰ _(BS,TX) for thebase station and V⁰ _(UE,TX)=V⁰ _(UE,RX) for the UE.

FIG. 11 illustrates control beams in a cell comprising DL and UL controlresource blocks and their associated beamforming weights. The set of DLand UL control resource blocks and their associated beamforming weightsset are collectively referred to as a Control Beam (CB) in a cell.Multiple sets of beamforming weights create radiation patterns coveringthe entire service area of the cell. One set of DL control resourceblocks and one set of UL control resource blocks are associated witheach of the beamforming weights set. Each cell has multiple controlbeams covering its entire service area. In the example of FIG. 11,control beam 0 (CB0) in cell 1100 comprises a set of DL control resourceblocks 1110, a set of UL control resource blocks 1120, and a set ofcorresponding beamforming weights or beamforming vectors (V⁰ _(BS,TX)=V⁰_(BS,RX)=V⁰ _(BS)). The base station allocates eight control beams fromCB0 to CB7 for cell 1100. CB0 is associated with beamforming vectors V⁰_(BS), CB1 is associated with beamforming vectors V¹ _(BS), and so on soforth. The collection of the eight beamforming vectors V⁰ _(BS) throughV⁷ _(BS) creates a radiation pattern covering the entire service area ofthe cell.

FIG. 12 illustrates control region, control region segment, and controlcycle of a control beam. The collection of all DL control resourceblocks associated with all control beams in a cell is referred to as theDL control region of a cell. DL control region may further be dividedinto DL control region segments. A DL control region segment comprisesDL control resource blocks associated with all or part of the controlbeams in a cell within a certain time period referred to as the controlcycle of the cell. Similarly, the collection of all UL control resourceblocks associated with all control beams in a cell is referred to as theUL control region of a cell. UL control region may further be dividedinto UL control region segments. A UL control region segment comprisesUL control resource blocks associated with all or part of the controlbeams in a cell within the control cycle of the cell. There is one DLcontrol segment and one corresponding UL control segment in a controlcycle of a cell. The control cycle of the cell may be pre-configured andknown to the UEs or dynamically configured and signaled to or blindlydetected by the UEs. The control cycle may vary over time.

In the example of FIG. 12, the top half of the diagram depicts the DLcontrol region having three DL control region segments. Each DL controlregion segment comprises DL control resource blocks for control beamsCB0, CB1, CB2, CB3, and CB4. The bottom half of the diagram depicts theUL control region having two UL control region segments. Each UL controlregion segment comprises UL control resource blocks for control beamsCB0, CB1, CB2, CB3, and CB4. A control cycle, e.g., from time T0 to T1,comprises one DL control region segment 1210 and one UL control regionsegment 1220.

FIG. 13 illustrates control region segment and control resource blockconfiguration. In the example of FIG. 13, a control region segmentcomprises control resource blocks for eight control beams from CB0 toCB7. The control region segment can occupy any time-frequency resourceblocks hardware allows for each CB. The different CBs can occupy theresource blocks in Time Division Multiplexed (TDM), in FrequencyDivision Multiplexed (FDM), in Code Division Multiplexed (CDM), inSpatial Division Multiplexed (SPD), or in any combination or mixture ofthe above multiplexing schemes.

FIG. 14 illustrates a preferred embodiment of DL and UL control resourceblock configuration. The configurations for DL control region segmentand UL control region segment need not to be the same. In the example ofFIG. 14, there are eight DL/UL control resource blocks for eight controlbeams CB0 to CB7 in a control cycle of a cell. In one DL control regionsegment, the DL control resource blocks for different control beams arepreferably Time Division Multiplexed (TDM) and contiguous in time. Asdepicted by block 1410, the DL control resource blocks for CB0 to CB7are multiplexed in time domain. Each control beam transmits at maximumpower to reach maximum range. On the other hand, in one UL controlregion segment, the UL control resource blocks for different controlbeams are preferably Spatial Division Multiplexed (SDM) in conjunctionwith other multiplexing schemes when a base station is equipped withmultiple RF chains. As depicted by block 1420, the UL control resourceblocks for CB0 to CB7 are multiplexed in spatial domain and in timedomain. The base station equipped with multiple RF chains can receivemultiple beams at the same time, and baseband digital processing canfurther mitigate inter-beam interference.

FIG. 15 illustrates an UL receiver having two RF chains for receivingtwo control beams simultaneously. In the example of FIG. 15, a basestation is equipped with an RF receiver having two RF chains RF0 andRF1. In UL transmission, the base station receives CB1 and CB5 at thesame time via RF0 and RF1, and then processes the received signal usinga digital baseband processing module 1510 to mitigate inter-beaminterference.

FIG. 16A illustrates embodiments with and without interleaved DL/ULcontrol resource configuration. In the top diagram of FIG. 16A, acontrol cycle comprises one DL control region segment 1610 and onecorresponding UL control region segment 1620. DL control region segment1610 comprises DL control resource blocks for four control beams CB0 toCB3. The DL control resource blocks for the four different control beamsare TDMed and contiguous in time. Similarly, UL control region segment1620 comprises UL control resource blocks for four control beams CB0 toCB3. The UL control resource blocks for the four different control beamsare TDMed and contiguous in time. In the bottom diagram of FIG. 16A, acontrol cycle comprises one DL control region segment 1630 and onecorresponding UL control region segment 1640. DL control region segment1630 comprises DL control resource blocks for four control beams CB0 toCB3. UL control region segment 1640 comprises UL control resource blocksfor four control beams CB0 to CB3. The DL control resource blocks andthe UL control resource blocks for the four different control beams areTDMed but not contiguous in time. In a special case, the DL and ULcontrol resource blocks are interleaved and alternate in time.

FIG. 16B illustrates one embodiment of control resource configurationwith different DL/UL duty cycles. In the top diagram of FIG. 16B, acontrol cycle comprises one DL control region segment 1650 and onecorresponding UL control region segment 1660. DL control region segment1650 comprises DL control resources for four control beams CB0 to CB3,which are TDMed and contiguous in time. Each DL control beam appeartwice in the control cycle. UL control region segment 1660 comprises ULcontrol resources for four control beams CB0 to CB3, which are TDMed andnot contiguous in time. Each UL control beam appear once in the controlcycle. As a result, the DL control beams have a shorter duty cycle thanthe UL control beams. In the bottom diagram of FIG. 16B, a control cyclecomprises one DL control region segment 1670 and one corresponding ULcontrol region segment 1680. DL control region segment 1670 comprises DLcontrol resources for four control beams CB0 to CB3. UL control regionsegment 1680 comprises UL control resources for four control beams CB0to CB3. The DL control resource blocks and the UL control resourceblocks for the four different control beams are TDMed but not contiguousin time. In a special case, every two DL control resource blocks areinterleaved by one UL control resource block. As a result, the DLcontrol beams have a shorter duty cycle than the UL control beams.

FIG. 17 illustrates embodiments of control cycles for different cells.In the top diagram of FIG. 17, the control cycles for different cellsare the same, e.g., cell-synchronous. The DL control region segments forcell1, cell2, and cell3 are time-aligned. With cell-synchronousconfiguration, a UE is able to perform measurements for control beamsfrom different cells during the same control region segment interval. Inthe bottom diagram of FIG. 17, the control cycles for different cellsare different, e.g., cell-non-synchronous. The DL control regionsegments for cell1, cell2, and cell3 are not time-aligned. Withcell-non-synchronous configuration, there is no inter-cell interferencebetween control beams from different cells.

FIG. 18 illustrates embodiments of control cycles in TDD and FDDsystems. In the top diagram of FIG. 18, the DL control region segmentsand the UL control region segments are interleaved in time in TDD or FDDmode. In the bottom diagram of FIG. 18, the DL control region segmentsand the UL control region segments may overlap or aligned in time in FDDmode.

Additional control resource blocks may be configured when thepreconfigured resources for control beams are insufficient. For DLcontrol beams, additional DL control resource blocks may be dynamicallyconfigured, pre-configured, or implicitly delivered from control beamidentification. The addition DL control resource blocks may have adifferent frame format, e.g., pilot signal is not modulated because itdoes not need to carry beam ID. For UL control beams, additional ULcontrol resource blocks may be dynamically configured, pre-configured,or implicitly delivered from control beam identification. The additionalUL control resource blocks may be allocated for contention based orgranted to a designated set of UEs. The additional UL control resourceblocks may have a different frame format, e.g., pilot signal is notmodulated because it does not need to carry UE ID.

FIG. 19 illustrates a control signaling procedure between a UE 1901 anda BS 1902 in a beamforming system in accordance with one novel aspect.In step 1910, UE 1901 tries to establish a connection with BS 1902. UE1901 waits and detects BS control beam transmission, which aretransmitted repeatedly and indefinitely. UE 1901 attempts to achievetime, frequency, and spatial synchronization with BS 1902, and acquiringrequired broadcast information for accessing the network. In step 1920,UE 1901 receives and detects control beam transmissions from BS 1902.For example, UE 1902 receives and detects four control beamtransmissions of CB#1 to CB#4 from BS 1902. In step 1930, UE 1901selects a control beam, e.g., control beam CB#2 for establishingconnection with BS 1902. UE 1901 first performs time and frequencysynchronization with BS 1902. Spatial synchronization is achieved afterthe UE selects the control beam for establishing the connection with theBS. UE 1901 then determines the UL control resources corresponding tothe selected control beam CB#2. Moderate array gain is provided via thecontrol beam, which partially compensates severe pathloss in mmWavechannel and thus facilitates detection operation at UE. In step 1940, UE1901 performs random access (RA) on the UL control resourcescorresponding to the selected control beam CB#2 for carrying essentialinformation to BS 1902 that is required for connection establishment.Via the random access, the BS is aware of which control beam ispreferred by the UE. The BS can reach the UE for completing theconnection establishment procedure by using the selected control beam.Moderate array gain is provided via the control beam that facilitates BSreception of UE random access. The UL control resources includededicated resource for random access and thus provide a better-protectedUL channel.

FIG. 20 is a flow chart of a method of control signaling from basestation perspective in a beamforming system in accordance with one novelaspect. In step 2001, a base station allocates a first sets of DLcontrol resource blocks for DL transmission to a plurality of userequipments (UEs) in a beamforming network. Each set of DL controlresource blocks is associated with a corresponding set of beamformingweights. In step 2002, the base station allocates a second sets of ULcontrol resource blocks for UL transmission from the UEs. Each set of ULcontrol resource blocks is associated with the same corresponding set ofbeamforming weights. In step 2003, the base station transmits cell andbeam identification information using a set of control beams. Eachcontrol beam comprises a set of DL control resource block, a set of ULcontrol resource block, and the corresponding set of beamformingweights. A collection of the beamforming weights of the set of controlbeams create a radiation pattern that covers an entire service area of acell.

FIG. 21 is a flow chart of a method of control signaling from userequipment perspective in a beamforming system in accordance with onenovel aspect. In step 2101, a user equipment (UE) receives control beamtransmission from a base station using a set of control beams in abeamforming network. Each control beam comprises a set of DL controlresource blocks, a set of UL control resource blocks, and an associatedset of beamforming weights. In step 2102, the UE selects a control beamfor establishing a connection with the base station. In step 2103, theUE performs random access with the base station using the selectedcontrol beam.

Pilot Signals in the Control Beams

FIG. 22 illustrates one example of pilot signals in a control beam in abeamforming system in accordance with one novel aspect. As illustratedearlier with respect to FIG. 1, for control signaling purpose, a set ofcoarse TX/RX control beams are provisioned by the base station in thecellular system. The set of control beams may be periodically configuredor occur indefinitely and repeatedly in order to be known to the UEs.The set of control beams covers the entire cell coverage area withmoderate beamforming gain. Each control beam broadcasts a minimum amountof beam-specific information similar to Master Information Block orSystem Information Block (MIB or SIB) in LTE. Each beam may also carryUE-specific control and/or data traffic. Each control beam transmits aset of known pilot signals for the purpose of initial time-frequencysynchronization, identification of the control beam that transmits thepilot signals, and measurement of radio channel quality for the controlbeam that transmits the pilot signals.

In the example of FIG. 22, a cell of a base station is covered by eightcontrol beams CB0 to CB7. Each control beam comprises a set of downlinkresource blocks, a set of uplink resource blocks, and a set ofassociated beamforming weights with moderate beamforming gain. Differentcontrol beams are time division multiplexed (TDM) in time domain. Forexample, a downlink subframe 2201 has eight DL control beams occupying atotal of 0.38 msec. An uplink subframe (not shown) also has eight ULcontrol beams occupying a total of 0.38 msec. The interval (a controlcycle) between two DL/UL subframes is 5 msec. The set of control beamsare lower-level control beams that provide low rate control signaling tofacilitate high rate data communication on higher-level data beams. Morespecifically, a set of pilot signals for each of the control beams istransmitted in each of the periodically configured time-frequencyresource blocks to facilitate the receiving devices to detect, identify,and synchronize to the control beams and perform the subsequent highrate data communication.

FIG. 23 illustrates a detailed example of pilot structures in controlbeams in a beamforming OFDM system. The interval (a control cycle)between two DL subframes in FIG. 23 is 5 msec, which contains 840 DLOFDM symbols. Each DL control region contains 128 OFDM symbols, and eachcontrol beam (e.g., CB0) within each control region contains 16 OFDMsymbols. For CB0, each resource block (e.g., resource block 2301)allocated for CB0 contains L=4 OFDM symbols along time domain and acertain number of subcarriers along frequency domain depending on systembandwidth and configuration.

Within each OFDM symbol, pilot symbols are inserted once every K=8subcarriers (or resource elements) for Rmax=320 times in one OFDM symbol(e.g., OFDM symbol 2311). The remaining subcarriers (or resourceelements) are for data symbols. The pilot symbols have a power-boostingfactor with respect to the data symbols. The pilot symbols have anoffset v_(m) with respect to the 0-th subcarrier. The pilot symbols spana sub-band or the entire band (K*Rmax≦N_(fft)). The R resource elementsin OFDM symbol 2311 are modulated by a signature sequence s_(m)[n] toidentify the control beam (CB0). The same OFDM symbol is repeated for Ltimes, e.g., for every OFDM symbols in resource block 2301, forming onepilot structure. Similar pilot structures are repeated for M times,indexed by repetition index m=0, 1 . . . M−1. For example, pilotstructure 2323 have a repetition index of m=2. The M pilot structures (Mrepetitions) together form the pilot part of CB0.

The M pilot structures have a similar structure but with a differentoffset v_(m) and/or a different signature sequence s_(m). The actualvalue of offset v_(m) and signature sequence s_(m) are based on therepetition index m. Potentially, the different offsets v_(m) resultingin a hopping pattern, while the different sequences s_(m) are generatedfrom circular delay-Doppler shifts of a base sequence. In one example,the different sequences s_(m) belong to a set of Zadoff-Chu sequenceswith different delays and chirping slopes. As a result, the R resourceelements in one OFDM symbol modulated by the signature sequence s_(m)[n]can be used to identify a specific control beam.

FIG. 24 illustrates control beam identification based on pilot signalsin a beamforming network 2400. Beamforming network 2400 comprises aplurality of cells. Each base station configures a set of control beamsto create a radiation pattern covering an entire service area of a cellfor pilot signal transmission. Each control beam of the cell isidentified by the pilot symbols having a hopping pattern v_(m) and asequence s_(m)[n]. Specifically, for the j-th control beam of the i-thcell, there is a corresponding identifier pair v_(m)(i,j) ands_(m)(i,j)[n] with some variations.

In the example of FIG. 24, there are nine cells (cell 0 to cell 8), andeach cell has eight control beams (CB0 to CB 7). In one example, thesame hopping pattern but different sequences are associated withdifferent control beams of the same cell. In another example, the samesequence but different hopping patterns are associated with differentcontrol beams of the same cell. In yet another example, sequences fordifferent control beams in the same cell belong to a set of sequencesderived from the same base sequence. Note that the same identifier pairmay be reused spatially.

FIG. 25 illustrates variations of pilot structures with additional OFDMsymbols. As depicted by the left diagram in FIG. 25, each resourceblocks allocated for a control beam contains six OFDM symbols along timedomain. The left most and the right most OFDM symbols are allocated fordata part, while part of the middle L=4 OFDM symbols are allocated forpilot part. Pilot symbols are inserted every K=8 subcarriers in each ofthe L=4 OFDM symbols and the same OFDM symbol is repeated for L=4 times.As depicted by the right diagram in FIG. 25, each resource blocksallocated for a control beam contains four OFDM symbols along timedomain. The left most and the right most OFDM symbols are allocated fordata part, while part of the middle L=2 OFDM symbols are allocated forpilot part. Pilot symbols are inserted every K=8 subcarriers in each ofthe L=2 OFDM symbols and the same OFDM symbol is repeated for L=2 times.In other words, L is configurable by the base station and additionalOFDM symbols can be padded before and/or after the pilot symbols tocarry additional data symbols.

FIG. 26 illustrates variations of pilot structures with guard time. Asillustrated in FIG. 26, between switching from one control beam toanother control beam, additional guard time can be inserted. Forexample, a guard interval is inserted between CB0 and CB1 to ensure thatdistinct transmissions for CB0 and CB1 do not interfere with oneanother.

FIG. 27 is a flow chart of a method of allocating resources for pilotsymbol transmission in a beamforming system in accordance with one novelaspect. In step 2701, a base station allocates a set of control resourceblocks in a beamforming OFDM network. The set of control resource blockscomprises periodically allocated time-frequency resource blocksassociated with a set of beamforming weights to form a control beam. Instep 2702, the base station partitions each resource block into a pilotpart and a data part. Each pilot part is divided into M pilot structuresand each pilot structure comprises L OFDM symbols. Pilot symbols areinserted once every K subcarriers for R times in each of the L OFDMsymbol to form the pilot part while data symbols are inserted in theremaining resource elements to form the data part. The variables M, L,K, and R are all positive integers. In step 2703, the base stationtransmits the pilot symbols and the data symbols via the control beam toa plurality of user equipments (UEs). The M pilot structures have asimilar structure with a different offset v_(m) and a different sequences_(m). Each control beam of a cell is identified by the pilot symbolshaving a hopping pattern based on v_(m) and a signature sequence s_(m).Specifically, for the j-th control beam of the i-th cell, there is acorresponding identifier pair v_(m)(i,j) and s_(m)(i,j)[n].

Variable Cyclic Prefix

As illustrated earlier with respect to FIG. 23, pilot symbols areinserted once every K subcarriers (or resource elements) for Rmax=320times in one OFDM symbol of each pilot structure. In order to facilitatepilot detection for the receiving device, the pilot symbols have apower-boosting factor with respect to the data symbols. In addition, thepilot symbols are repeated for L times in each pilot structure. The Lrepetitions can be implemented in different ways.

FIG. 28 illustrates different embodiments of OFDM symbols with Lrepetitions in time domain. In the top diagram 2810 of FIG. 28, the Lrepetitions are implemented in a traditional way using L OFDM symbols.That is, the resource elements (once every K subcarriers) in one OFDMsymbol are modulated by a signature sequence s_(m)[n] of the pilotsignal. The same OFDM symbol is repeated for L times forming one pilotstructure. Diagram 2810 illustrates a time domain representation of theL=4 OFDM symbols after performing IFFT with a normal FFT size of N_(fft)and each OFDM symbol has a normal CP length of Ncp. In one example,N_(fft)=1024, and the N_(CP)=128.

In the bottom diagram 2820 of FIG. 28, the L repetitions are implementedusing one long OFDM symbol with longer FFT size CP length. That is, theresource elements (once every L*K subcarriers) in one long OFDM symbolare modulated by a signature sequence s_(m)[n] of the pilot signal.Diagram 2820 illustrates a time domain representation of the long OFDMsymbol after performing IFFT with a FFT size of L*N_(fft) and the OFDMsymbol has a CP length of L*Ncp. In one example, L*N_(fft)=4096, andL*N_(CP)=512. With long FFT size and long CP length, the same pilotsymbols are repeated L times with phase continuity across the Lrepetitions in the long OFDM symbol. Note that if normal size FFT isperformed on the original OFDM boundary, then phase shift rotation isneeded on the pilot symbols to implement the bottom diagram 2820.

FIG. 29 illustrates pilot structures with variable cyclic prefix lengthand variable length FFT. At the transmitting side 2901, a pilot signalis first converted by a serial to parallel converter, pilot symbols areinserted to a resource block of a control beam and converted fromfrequency domain signal to time domain signal by applying IFFT, and thenadded with cyclic prefix before control beam transmission. At thereceiving side 2902, the receiver operates on the received time domainsignal by performing FFT to reconstruct the pilot structure. The cyclicprefix is first removed from the received signal, then converted fromtime domain signal back to frequency domain signal by applying FFT, andconverted by a parallel to serial converter. Pilot symbols are extractedto recover the pilot signal.

Diagram 2910 of FIG. 29 is a frequency domain representation for pilotstructure with index m=0, and diagram 2930 is a corresponding timedomain representation of the OFDM symbols for pilot structure with indexm=0. As shown in diagram 2910, pilot symbols are inserted once every K=8subcarriers for the first OFDM symbol. The same OFDM symbol is thenrepeated for the second OFDM symbol. As shown in diagram 2930, the firsttwo OFDM symbols are applied with FFT size of N_(fft) with a CP lengthof N_(CP). Starting from the next OFDM symbol in the same pilotstructure m=0, pilot symbols are inserted once every K=2*8=16subcarriers as depicted by 2910. The third OFDM symbol is applied withFFT size of 2*N_(fft) with a CP length of 2*N_(CP) as depicted by 2930.Similarly, diagram 2920 of FIG. 29 is a frequency domain representationfor pilot structure with index m=1, and diagram 2940 is a correspondingtime domain representation of the OFDM symbol for pilot structure withindex m=0. As shown in diagram 2920, pilot symbols are inserted onceevery K=4*8=32 subcarriers for the OFDM symbol. As shown in diagram2940, the OFDM symbol is applied with FFT size of 4*N_(fft) with a CPlength of 4*N_(CP).

In beamforming networks, the delay spread is larger for wider beambecause of more multi-paths in the channel and results in greater chanceto pull in paths with longer delays. Some UEs may choose to use wider RXbeam to search for control beams, and thus have longer delay spreads inthe received signal. Some UEs may not even support beamforming. Longerdelay spread needs longer CP length. With certain control beam resourceblocks configured to support variable-length FFT with variable-length CPlength in different pilot structures, UEs with larger delay spread canreceived their control data using longer FFT size and longer CP length.Note that the pilot symbols remain unchanged across the L repetitions.Therefore, pilot symbols are always processed at the largest FFT size(e.g., L*N_(fft)), or at its equivalent regular FFT size (e.g., N_(fft))with appropriate phase rotations/shifts. Furthermore, constant power ismaintained for the pilot symbols across the L repetitions.

FIG. 30 is a flow chart of supporting variable CP length for pilotsignal transmission in a beamforming network in accordance with onenovel aspect. In step 3001, a base station allocates time-frequencyresource blocks in a beamforming OFDM network for control beam (CB)transmission. In step 3002, the base station partitions each resourceblock into a pilot part and a data part. The pilot part comprises Mpilot structures and each pilot structure comprises a number of OFDMsymbols in time domain and a number of subcarriers in frequency domain.In step 3003, the base station inserts pilot symbols of a pilot signalin each OFDM symbol in the pilot part. The pilot symbols are repeatedfor L times in each pilot structure, and each pilot structure is appliedby one or more Inverse Fast Fourier Transfers (IFFTs) with correspondingone or more variable cyclic prefix (CP) lengths for CB transmission. Mand L are positive integers. In one embodiment, a pilot structurecomprises L OFDM symbols, the L repetitions are implemented by an IFFTof length N_(fft) for L times, and each OFDM symbol has a cyclic prefix(CP) length of N_(CP). In another embodiment, a pilot structurecomprises one OFDM symbol, the L repetitions are implemented by an IFFTof length (L×N_(fft)), and the OFDM symbol has a cyclic prefix (CP)length of (L×N_(CP)).

FIG. 31 is a flow chart of supporting variable CP length for pilotsignal reception in a beamforming network in accordance with one novelaspect. In step 3101, a user equipment (UE) receives control beamtransmission from a base station in a beamforming OFDM network. The UEreceives a time domain signal from pilot symbols that are transmittedover periodically allocated time-frequency resource blocks of a controlbeam. In step 3102, the UE processes the time domain signal by removingone or more cyclic prefixes (CPs) with a variable CP length andperforming corresponding one or more variable-length Fast FourierTransfers (FFTs) to reconstruct a pilot part of a resource block,wherein the pilot part comprises M pilot structures and each pilotstructure comprises a number of OFDM symbols in time domain and a numberof subcarriers in frequency domain. In step 3103, the UE extracts thepilot symbols from each pilot structure. The pilot symbols are repeatedfor L times in each pilot structure, and M and L are positive integers.In one embodiment, a pilot structure comprises L OFDM symbols, the UEextracts the L repetitions by performing an FFT of length N_(fft) for Ltimes, and each OFDM symbol had a cyclic prefix (CP) length of N_(CP).In another embodiment, a pilot structure comprises one OFDM symbol, theUE extracts the L repetitions by performing an FFT of length(L×N_(fft)), and the OFDM symbol has a cyclic prefix (CP) length of(L×N_(CP)).

Detection Procedure

FIG. 32 illustrates a three-stage pilot signal detection procedure inaccordance with one novel aspect. As illustrated earlier with respect toFIG. 23, pilot signals are transmitted via control beams in a cell usingperiodically allocated radio resource blocks. For DL pilot transmission,a base station allocates radio resource blocks (e.g., resource block3210) and insert pilot symbols onto each pilot structure (e.g., pilotstructure 3220). Within each OFDM symbol, pilot symbols are insertedonce every K=8 subcarriers (or resource elements) for Rmax=320 times inone OFDM symbol. The pilot symbols have a power-boosting factor withrespect to the data symbols. The pilot symbols have an offset v_(m) withrespect to the 0-th subcarrier. The pilot symbols span a sub-band or theentire band (K×Rmax≦N_(fft)). The R resource elements in each OFDMsymbol are modulated by a signature sequence s_(m)[n] to identify acontrol beam. The same OFDM symbol is repeated for L times, e.g., forevery OFDM symbols in the resource block, forming one pilot structure.Similar pilot structures are repeated for M times, indexed by repetitionindex m=0, 1 . . . M−1.

The pilot signal for (cell i, control beam CB j) can be represented by:

${p^{({i,j})}(t)} = {\sum\limits_{m = 0}^{M - 1}\;{{s_{m}^{({i,j})}\left( {t - {mLT}_{0}} \right)}e^{j\; 2{\pi v}_{m}^{({i,j})}{f_{s}{({t - {mLT}_{0}})}}}}}$Where

-   -   s_(m) ^((i,j))(t) is the time domain equivalent of the pilot        signal for the j-th control beam of the i-th cell in the m-th        repetition.    -   L is the number of OFDM symbols in each pilot structure.    -   m=0 . . . M−1 is the repetition index of each pilot structure.    -   v_(m) ^((i,j)) is the offset with respect to the 0-th        subcarrier.    -   T₀ is the regular OFDM symbol length (Ts) plus the regular CP        length T_(CP).

The received signal at the UE through delay-Doppler channel can berepresented by:

${r(t)} = {\sum\limits_{i,j}\;{\int{{p^{({i,j})}\left( {t - \tau} \right)}{e^{j\; 2\pi\;{vt}} \cdot {h^{({i,j})}\left( {\tau,v} \right)}}d\;{\tau d}\; v}}}$Where

-   -   p^((i,j)) is the pilot signal for the j-th control beam in the        i-th cell.    -   h^((i,j)) is the channel response for the j-th control beam in        the i-th cell.

Based on the received signal r(t) from the control beam transmission,the receiving device (UE) needs to detect the presence of the pilotsignal, e.g., identify the (cell, CB) ID and achieve time-frequencysynchronization based on r(t). An exemplary three-stage pilot detectionapproach with reduced complexity is proposed. In stage-1 detection (step3201), the UE detects the presence of control beams and performs coarsetime-frequency offset estimation. In stage-2 detection (step 3202), theUE detects control beam resource boundaries. In stage-3 detection, theUE first performs signature sequence correlation and beam identification(step 3203), and then performs fine time-frequency synchronization andchannel estimation (step 3204).

FIG. 33 illustrates a stage-1 control beam detection and coarsetime-frequency estimation. A stage-1 detection detects presence of anycontrol beam and its coarse resource block boundary and estimates thecoarse time and carrier frequency offset. The detector calculates thesliding DFT of the extended OFDM symbols (the sliding windows may or maynot overlap). Energy is summed over sub-carriers in which pilot symbolsare inserted for each hypothesized frequency offset.

As illustrated in FIG. 33, UE receives a time-domain signal, which iscarried by L*N samples in each pilot structure. The received time-domainsignal is then converted to a frequency domain signal via DiscreteFourier Transfer (DFT). At each time instance t, potential pilot symbolsare extracted once every K*L subcarriers (e.g., resource elements ortones) from a total of L*N subcarriers with an offset i. The receiverapplies a sliding DFT plus combining algorithm in detecting the presenceof any control beam and pilot symbols based on energy detection. Inother words, the amplitude (energy) of the potential pilot tones aresummed up for time instance t and offset i, and the receiver chooses thebest (t,i) such that the amplitude reaches the maximum. Morespecifically, at time instance t and offset i=0 . . . (KL−1), theamplitude summation of pilot tones can be represented by:Amp_(t) ₀ _(,0)=Σ|·|², Amp_(t) ₀ _(,KL−1)=Σ|·|² (for time t0)Amp_(t) _(n) _(,0)=Σ|·|², Amp_(t) _(n) _(,KL−1)=Σ|·|² (for time tn)

Once the time and frequency index (t,i) is chosen, then the receiverdetermines that the coarse central frequency CFO=i*(1/KL), and thecoarse OFDM boundary is at time t. Note that if the pilot tones have apower boosting as compared to data tones, the energy detection methodmay be more accurate.

FIG. 34 illustrates a stage-2 control beam reference block boundarydetection. A Stage-2 detector detects the coarse boundary (up to awindow of uncertainty) of the control beam resource block boundarydetected in stage-1 after the correction of frequency offset estimatedin stage-1. Stage-2 detection is similar to stage-1 with the uncertaintyin frequency offset removed, potentially longer extended OFDM symbols,and finer sliding window resolution. During the stage-2 energydetection, the presence of more control beams and their finertime-frequency synchronization can be achieved. The receiver applies asimple sliding DFT with energy detection of coherently accumulated pilotsymbols after coarse CFO correction. The receiver is then able todetermine a small fraction of OFDM symbol and sub-carrier for finertime-frequency synchronization.

When the pilot symbols are inserted every K sub-carriers in an OFDMsymbols, the corresponding time domain signal will be K repetitions of acertain length (N_(fft)/K) sequence related to the complex values of thepilot symbols. Each one of those repetitions is a pilot segment. In theexample of FIG. 34, the cyclic prefix is of length N_(fft)/K. Therefore,there are K+1 pilot segments in each OFDM symbol including its cyclicprefix. Because there are L repetitions in a pilot structure, there area total of (K+1)*L pilot segments. At each time instance, all samples ofthe pilot segments are summed up to output an absolute accumulation(energy), e.g., AccOutt0 for time instance t0, and AccOutt1 for timeinstance t1, and so on so forth. For example, at time t1, theaccumulation of all samples of the pilot segment can be represented byAccOutt1=RxS₁+ . . . +RxS_((K+1)*L). The receiver then choose the timeinstance with the maximum accumulation for time domain synchronization.The L times repetition raises detection metric for pilot symbols by10*log₁₀(L)dB. Some control data symbols may also be repeated L times toyield sufficient SNR level for cell edge UEs, and the base station canavoid false alarm by limiting such resource mapping or distributing suchresource mapping randomly. Pilot power boosting with respect to datasymbols further improves detection performance.

FIG. 35 illustrates a stage-3 sequence correlation and beamidentification and finest time-frequency synchronization. From theoutcome of the stage-2 detection, a search interval is determined, whichis in the order of the cyclic prefix length. During a search interval,all the pilot segments are summed up by the receiver: RxS=RxS₁+ . . .+RxS_((K+1)*L). The receiver then correlates the received signal RxSwith all possible pilot sequences p(i,j) for all i and j. Control beam jin cell i is detected if it has the maximum correlation out in thesearch interval. Finest time and frequency synchronization is thenperformed. For example, once the strongest sequence and its associatedcontrol beam are detected, the stage-3 process in FIG. 35 can beperformed for that particular sequence over finer hypotheses of the timeand frequency offset interval to achieve the finest time and frequencysynchronization.

FIG. 36 is a flow chart of a method of pilot signal detection based oncontrol beam transmission in a beamforming network in accordance withone novel aspect. In step 3601, a user equipment (UE) receives controlbeam transmissions from a base station in a beamforming OFDM network. Apilot signal is transmitted over periodically allocated time-frequencyresource blocks of a control beam in a cell. In step 3602, the UEprocesses pilot symbols carried in a pilot part of a resource block, thepilot part comprises M pilot structures and each pilot structurecomprises L OFDM symbols in time domain and R subcarriers in frequencydomain. The pilot symbols are inserted once every K subcarriers for Rtimes in each OFDM symbol, and M, L, R, and K are positive integers. Instep 3603, the UE detects the control beam and the pilot signal based onthe control beam transmission.

In one embodiment, a three-stage pilot detection procedure is performed.In stage-1, the UE detects an existence of the control beam byperforming a sliding Discrete Fourier Transform (DFT) and therebyestimating a coarse time-frequency offset. It involves energy detectionby selecting a time instance and a frequency offset to achieve a maximumcombined energy. In stage-2, the UE detects a time-frequency resourceblock boundary of the control beam. In involves performing a sliding DFTwith energy detection of accumulated pilot symbols of a fraction of OFDMsymbols and sub-carriers. In stage-3, the UE detects the pilot signaland identifying the control beam and performing fine time-frequencysynchronization and channel estimation. It involves sequence correlationwith all possible pilot sequences. The detected control beam has amaximum correlation during a search interval determined by the stage-2detection.

Although the present invention has been described in connection withcertain specific embodiments for instructional purposes, the presentinvention is not limited thereto. Accordingly, various modifications,adaptations, and combinations of various features of the describedembodiments can be practiced without departing from the scope of theinvention as set forth in the claims.

What is claimed is:
 1. A method comprising: allocating time-frequencyresource blocks by a base station in a beamforming OFDM network forcontrol beam (CB) transmission, wherein each allocated time-frequencyresource block comprises a plurality of OFDM symbols in time domain anda plurality of subcarriers in frequency domain; partitioning eachresource block of the allocated time-frequency resource blocks into apilot part and a data part by the base station, wherein the pilot partcomprises M pilot structures and each pilot structure of the M pilotstructures comprises a number of the plurality of OFDM symbols in timedomain and a number of the plurality of subcarriers in frequency domain;and inserting pilot symbols of a pilot signal in each of the number ofthe plurality of OFDM symbols in the pilot part by the base station bymodulating each of the M pilot structures with a signature sequence,wherein the pilot symbols are repeated for L times in each of the Mpilot structures, wherein each of the M pilot structures is applied bymultiple variable-length Inverse Fast Fourier Transfers (IFFTs) havingvarying IFFT window sizes with corresponding multiple varying cyclicprefix (CP) lengths for CB transmission, wherein both the pilot symbolsand data symbols are multiplexed in subcarriers of each of the number ofthe plurality of OFDM symbols that employs the multiple variable-lengthIFFTs, and wherein M and L are positive integers.
 2. The method of claim1, wherein a pilot structure of the M pilot structures comprises L OFDMsymbols, wherein the L repetitions are implemented by an IFFT of themultiple variable-length IFFTs of length Nfft for L times, wherein eachof the L OFDM symbols has a cyclic prefix (CP) length of N_(CP), andwherein N_(fft) and N_(CP) are integers.
 3. The method of claim 2,wherein the pilot symbols of the L repetitions are phase shiftedresulting in phase continuity across the L repetition.
 4. The method ofclaim 1, wherein a pilot structure of the M pilot structures comprisesone OFDM symbol, wherein the L repetitions are implemented by an IFFT oflength (L×N_(fft)), wherein the one OFDM symbol has a cyclic prefix (CP)length of (L×N_(cp)), and wherein N_(fft) and N_(CP) are integers. 5.The method of claim 1, wherein each of the M pilot structures has asimilar structure, but has a different offset with respect to the 0-thsubcarrier of the number of the plurality of subcarriers.
 6. The methodof claim 1, wherein a first IFFT of the multiple variable-length IFFTswith a first IFFT window size and a first CP length is applied for apilot structure, and wherein a second IFFT of the multiplevariable-length IFFTs with a second IFFT window size and a second CPlength is applied for the same pilot structure.
 7. The method of claim1, wherein a first IFFT of the multiple variable-length IFFTs with afirst IFFT window size and a first CP length is applied for a firstpilot structure, and wherein a second IFFT of the multiplevariable-length IFFTs with a second IFFT window size and a second CPlength is applied for a second pilot structure.
 8. The method of claim1, wherein the resource block is configured to support the multiplevariable-length IFFTs, and wherein subcarrier spacing changes based onFFT length.
 9. The method of claim 1, wherein the pilot symbols remainunchanged across the L repetitions in a pilot structure of the M pilotstructures.
 10. The method of claim 1, wherein constant power is appliedfor the pilot symbols across the L repetitions in a pilot structure ofthe M pilot structures.
 11. A method comprising: receiving control beamtransmission by a user equipment (UE) from a base station in abeamforming OFDM network, wherein the UE receives a time domain signalfrom pilot symbols transmitted over periodically allocatedtime-frequency resource blocks of the received control beam, whereineach allocated time-frequency resource block comprises a plurality ofOFDM symbols in time domain and a plurality of subcarriers in frequencydomain; processing the received time domain signal by the UE by removingmultiple cyclic prefixes (CPs) with a variable CP length and performingcorresponding multiple variable-length Fast Fourier Transfers (FFTs) toreconstruct a pilot part of a resource block, wherein the pilot partcomprises M pilot structures and each of the M pilot structurescomprises a number of the plurality of OFDM symbols in time domain and anumber of the plurality of subcarriers in frequency domain, and whereina data part of the same resource block is also reconstructed by themultiple variable-length FFTs; and extracting the pilot symbols fromeach of the M pilot structures by the UE by demodulating each of the Mpilot structures to a signature sequence, wherein the pilot symbols arerepeated for L times in each of the M pilot structures, and wherein Mand L are positive integers.
 12. The method of claim 11, wherein a pilotstructure of the M pilot structures comprises L OFDM symbols, whereinthe UE extracts the L repetitions by performing an FFT of the multiplevariable-length FFTs of length N_(fft) for L times, wherein each of theL OFDM symbols has a cyclic prefix (CP) length of N_(CP), and whereinN_(fft) and N_(CP) are integers.
 13. The method of claim 12, wherein thepilot symbols of the L repetitions are phase shifted resulting in phasecoherency across the number of sub-carriers in the pilot part.
 14. Themethod of claim 11, wherein a pilot structure of the M pilot structurescomprises one OFDM symbol, wherein the UE extracts the L repetitions byperforming an FFT of length (L×N_(fft)), wherein the one OFDM symbol hasa cyclic prefix (CP) length of (L×N_(CP)), and wherein N_(fft) andN_(CP) are integers.
 15. The method of claim 11, wherein the UE performsa first FFT of the multiple variable-length FFTs with a first FFT windowsize and a first CP length for a pilot structure, and wherein the UEperforms a second FFT of the multiple variable-length FFTs with a secondFFT window size and a second CP length for the same pilot structure. 16.The method of claim 11, wherein the UE performs a first FFT of themultiple variable-length IFFTs with a first FFT window size and a firstCP length for a first pilot structure, and wherein the UE performs asecond FFT of the multiple variable-length IFFTs with a second FFTwindow size and a second CP length for a second pilot structure.
 17. Auser equipment (UE), comprising: a radio frequency (RF) receiver thatreceives control beam transmission by a user equipment (UE) from a basestation in a beamforming OFDM network, wherein the UE receives a timedomain signal from pilot symbols transmitted over periodically allocatedtime-frequency resource blocks of the received control beam, whereineach allocated time-frequency resource block comprises a plurality ofOFDM symbols in time domain and a plurality of subcarriers in frequencydomain; a processor that processes the received time domain signal byremoving multiple cyclic prefixes (CPs) with a variable CP length andperforming corresponding multiple variable-length Fast Fourier Transfers(FFTs) to reconstruct a pilot part of a resource block, wherein thepilot part comprises M pilot structures and each pilot structure of theM pilot structures comprises a number of the plurality of OFDM symbolsin time domain and a number of the plurality of subcarriers in frequencydomain, and wherein a data part of the same resource block is alsoreconstructed by the multiple variable-length FFTs; and a symbolextractor that extracts the pilot symbols from each of the M pilotstructures by demodulating each of the M pilot structures to a signaturesequence, wherein the pilot symbols are repeated for L times in each ofthe M pilot structures, and wherein M and L are positive integers. 18.The UE of claim 17, wherein a pilot structure of the M pilot structurescomprises L OFDM symbols, wherein the UE extracts the L repetitions byperforming an IFFT of length N_(fft) for L times, wherein each of the LOFDM symbols has a cyclic prefix (CP) length of N_(CP), and whereinN_(fft) and N_(CP) are integers.
 19. The UE of claim 18, wherein thepilot symbols of the L repetitions are phase shifted resulting in phasecoherency across the number of subcarriers of the pilot part.
 20. The UEof claim 17, wherein a pilot structure of the M pilot structurescomprises one OFDM symbol, wherein the UE extracts the L repetitions byperforming an IFFT of length (L×N_(fft)), wherein the one OFDM symbolhas a cyclic prefix (CP) length of (L×N_(CP)), and wherein N_(fft) andN_(CP) are integers.
 21. The UE of claim 17, wherein the UE performs afirst FFT of the multiple variable-length FFTs with a first FFT windowsize and a first CP length for a pilot structure, and wherein the UEperforms a second FFT of the multiple variable-length FFTs with a secondFFT window size and a second CP length for the same pilot structure. 22.The UE of claim 17, wherein the UE performs a first FFT of the multiplevariable-length FFTs with a first FFT window size and a first CP lengthfor a first pilot structure, and wherein the UE performs a second FFT ofthe multiple variable-length FFTs with a second FFT window size and asecond CP length for a second pilot structure.